The GDNF Family of Ligands (GFLs) signal through a receptor complex composed of the Ret tyrosine kinase and a member of the GFRalpha coreceptor family. Studies of mice deficient in various components of the GFL system have revealed that GFL-signaling is important for the development and function of a wide variety of neuronal populations as well as the urogenital system. Deficits in these mutant animals lead to the conclusion that Ret activity supports progenitor cell proliferation, cell migration and axonal projection. Furthermore, both activating and inactivating mutations in Ret result in human disease, highlighting the importance of understanding the signal transduction pathways that normally transmit GFL-mediated signals to their cellular targets. Our central hypothesis is that signals emanating from specific Ret isoforms and through individual Ret tyrosine residues specifically regulate these physiologic processes. To test this hypothesis, we plan to examine Ret signaling in mice expressing specific Ret isoforms, either wild type or those containing mutations that eliminate specific protein interactions. The phenotypes of these mice will be examined for deficits in the nervous system that highlight the contributions of specific domains and tyrosine residues to the physiologic functions of Ret. In vitro models will be used to study the effects of specific Ret mutants and/or alterations in adaptor protein activities to further define the effects of Ret signaling on migration, proliferation and axonal projection. Finally, spermatogenesis has recently been discovered to depend on Ret activity. The availability of Ret mutants that survive the neonatal period will be exploited to explore this newly discovered role of Ret. The implementation of testicular transplantation technology will allow us to examine spermatogenesis in mouse mutants that die during the neonatal period. This technique will be used to examine spermatogenesis in GDNF, GFRalpha1 and Ret-deficient animals, as well as other Ret mutants we generate that might die at birth. Through these studies, new insights into the molecular mechanisms of GFL-mediated signaling will be forthcoming.